The object is to understand DNA repair and mutagenesis at a molecular level. UV-reactivation is a repair process that is highly mutagenic for the DNA that is repaired and is conveniently studied in microbial systems. Bacterial viruses (such as phi X174) inactivated by ultraviolet radiation are reactivated by infection of bacteria that have received a small dose of radiation; over 10% of the reactivated viruses are mutagenized. A theory is presented to explain several known facts and paradoxes about the mutagenic specificity of UV-reactivation, and a further test of the theory is proposed. It is predicted that a substantial proportion of the mutants induced by UV-reactivation will contain the double mutation CC yields TT, ordinarily a very unusual occurrence. Such a mutation can be observed in several ways. From the genetic code we know that in some instances the mutation of CC yields TT will result in the amino acid substitution proline yields phenylalanine in the product of the mutant gene. Amino acid analyses of mutant proteins will be used to test this prediction. The mutation of the tryptophan codon to an ochre nonsense codon in one step is another test of the theory. The most direct test proposed is to analyze the nucleotide sequence of the mutant DNA. In addition to studying the mutagenic repair of lesions induced by ultraviolet light, the repair of chemically induced lesions will also be examined.